Sewing machine drive control device

ABSTRACT

A sewing machine drive control device comprising a clutch member mounted on the main shaft of a sewing machine to be controlled by the control device and adapted to transmit power from a prime mover to the main shaft and interrupt the power transmission to the shaft, a control lever for operating the clutch member as the control lever moves in one or the other direction, a control cam having cam faces in relation to rotation and stoppage of the main shaft, respectively and interlocked with the main shaft for rotating at a speed less than that of the main shaft, and an interlocking member between the control cam and control lever for controlling the operation of the control lever through the cam faces on the control cam.

United States Patent 1 91 Fujii 1 1 Sept. 30, 1975 [54] sEwiNt; MACHINE DRIVE CONTROL DEVICE [75] Inventor: Ikunori Fujii, Fuchu, Japan [73] Assignec: Tokyo .luki Kogyo Kabushiki Kaisha, Japan [22] Filed: Sept. 19, 1973 [21] Appl. No.: 398,782 1 [52] U.S. Cl 112/67; 112/219 R; 112/252 [51] Int. Cl D05b 3/00 [58] Field of Search.. 112/67. 219 R, 219 A, 219 B. 112/220, 87

Primary E.\'aminer-H. Hampton Hunter Attornqv, Agent, or Firn1Wenderoth, Lind & Ponack [57] ABSTRACT A sewing machine drive control device comprising a clutch member mounted on the main shaft of a sewing machine to be controlled by the control device and adapted to transmit power from a prime mover to the main shaft and interrupt the power transmission to the shaft, a control lever for operating the clutch member as the control lever moves in one or the other direction, a control cam having cam faces in relation to rotation and stoppage of the main shaft, respectively and interlocked with the main shaft for rotating at a speed less than that of the main shaft, and an interlocking member between the control cam and control lever for controlling the operation of the control lever through the cam faces on the control cam.

3 Claims, 10 Drawing Figures US. Patent. Sept. 30,1975 Sheet 1 of7 3,908,568

US. Patent Sept. 30,1975 Sheet2of7 3,908,568

US. Patent Sept. 30,1975 Sheet 3 of7 3,908,568

Sheet 4 0f US. Patent Sept. 30,1975

US. Patent Sept. 30,1975 Sheet 5 of7 3,908,568

US. Patent Sept. 30,1 975 Sheet60f7 3,908,568

w O@ a 5, 0m 6R mm um firvm 5 5 U.S. Patent Sept. 30,1975 Sheet 7 of7 SEWING MACHINE DRIVE CONTROL DEVICE BACKGROUND OF THE INVENTION This invention relates to a sewing machine drive control device and more particularly, to a device for controlling the operation of a cyclically operated sewing machine which initiates its cyclic sewing operation by a start control manipulation and stops in a predetermined position after a predetermined number of stitches have been formed.

As one of the prior art sewing machine drive control devices of the above type, the control device as disclosed in US. Pat. No. 3,216,381 has been known. However, in the control device of this US. patent, as

the lever 106 (see FIG. 4 of the US. patent) is pulled with a strong force when the sewing machine in which the control device is incorporated is in its rest position, the sewing machine is promptly driven at a high speed. Therefore, the stationary parts of the pulley and main shaft and the crank and the like parts mounted on the main shaft for driving various sewing machine mechanisms are suddenly subjected to high load resulting in loosening, breakage arid/or premature wear of such parts. And when the sewing machine is suddenly driven at a high speed from its rest position, the thread may come off the hole in the needle and as a result, a desired seam may not be formed. Furthermore, in the control device of the aforementioned US. patent, when the sewing machine which is being driven at a high speed is suddenly stopped, there may be generation of noise and/or impact in the parts for controlling the driving of the sewing machine. Such a disadvantage is due to the fact that the shifting of the belt shifter 25 for stopping the sewing machine is effected by engagement of the tooth 100 on the lever 99 with the steps 96, 97 and 98 on the lever94. As a result, each time when the tooth I disengages from the steps, noise and/or impact are generated. And the tooth 100 and steps are readily subjected to wear and/or damage to the degree that positive control on the sewing machine over its ser vice life is rendered very difficult.

SUMMARY OF THE INVENTION Therefore, the object of the present invention is to eliminate the disadvantages inherent in the prior art sewing machine drive control device referred to above.

In brief, the sewing machine drive control device of the invention comprises a clutch member mounted on an outer extension of the machine shaft of a sewing machine in which the control device is incorporated for transmitting power from a prime mover to the main shaft or interrupting such power transmission, a control level for controlling the operationof the clutch member, a control cam adapted to rotatein response to the rotation of the main shaft, an interlocking member'for interlocking between the control lever and control cam and an operation lever for operating'the interlocking member upon being actuated by a manual actuator.

With the above construction of the sewing machine drive control device of the invention, by pulling the operation lever, control on- ,the start, speed change and stoppage of the sewing. machine can be effected through the control cam whereby drive and stoppage control of the machine is made positive. Furthermore, as a roller on the interlocking member moves smoothly along the cam faces on the control cam, control on the sewing machine is smoothly and quietly-effected. Still lowed, noise and/or impact which will be generated as i the driving of the sewing machine is controlled can be minimized thereby to insure a longer service life of the sewing machine. 1

Therefore, one principal object of the present invention is to provide a sewing machine drive control device whereby drive control on a sewing machine can be precisely effected when the sewing machine is driven at a high speed from its rest position and the sewing machine is stopped from its high speed driven position.

Another object of the present invention is to provide a sewing machine drive control device whereby drive control on a sewing machine can be smoothly effected with a minimum impact generated in parts of the control mechanism.

The above and other objects and attendant advantages of the present invention will be more readily apparent to those skilled in the art from a reading of the following description in conjunction with the accompa nying drawings which show one preferred embodiment of the invention for illustration purpose only, but not for limiting the scope of the same in any way.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a front elevational view of a preferred 'embodiment of device of the present invention. as mounted on the rear portion of a sewing machine when the machine is in its rest position;

FIG. 2 is an end elevational view of the control device as seen at the right-hand end of FIG. 1;

FIGS. 3, 4 and 5 are fragmentary views showing the device in its various positions in which FIG. 3 shows the positions of elements of the device when the sewing machine is in its rest position, FIG. 4 shows the positions of the elements when the sewing machine is driven at a low speed and FIG. 5 shows the positions of the elements when the sewing machine is driven at a high speed;

FIGS. 3A, 5A and 5B are fragmentary views showing the thread cutting mechanism in its various positions in which FIG. 3A shows the position of the thread cutting mechanism when the sewing machine is in its rest position, FIG. 5A shows the position of the thread cutting mechanism when the sewing machine is driven at a low or high speed and FIG. 5B shows the position of the thread cutting mechanism when the rotational movement from the prime mover to the main shaft of the sewing machine is interrupted;

FIG. 6 is a perspective view of the complete thread cutting mechanism; and

FIG. 7 is a perspective view of the control device] shown in FIG. 1.

PREFERRED EMBODIMENT OF THE INVENTION The illustrated embodiment of the invention will be described as being applied to a cyclically operable sew ing machine such as bar tracking and buttonhole sew ing machines.

Referring to the accompanying drawings and more particularly, to FIGS. 1,2 and 7, reference numeral 1 denotes the sewing machine main shaft which is rotatably supported in the framework 2 of the sewing machine. The main shaft 1 extends outwardly of the framework 2 and the shaft extension suitably supports thereon a thread cutting cam 3, a stopper 4, a clutch assembly 5 and a pulley 6. The pulley 6 is rotated from an electric motor (not shown) through a suitable conventional power transmission unit (not shown) and is also axially movable along the main shaft extension. Furthermore, the pulley 6 is normally resiliently urged rightwards as seen in FIG. 1. A control lever 8 is rotatably supported in the framework 2 by means of a shaft 7 the axis of which is parallel to that of the main shaft 1. A stopper shaft 13 is provided at the upper end with a stopper pawl 12 which is adapted to engage into and disengage out of a notch 11 formed in the stopper 4.

The above-mentioned stopper shaft 13 is vertically movably supported in support blocks 9 and 10 which also support the control lever 8. A spring 58 is disposed on the upper end portion of the stopper shaft 13 and held under compression between a member 57 fixedly secured to the upper end of the shaft 13 and the support block 10. A resilient clutch plate 17 is secured to the control lever 8 by suitable means. The upper end of the clutch plate 17 is provided with an outwardly extending portion 15 and an extension 16 which extends from the portion 15 at right angles thereto. A clutch ball 14 is held between the clutch portion and extension 15, 16 and the pulley 6. A bolt 21 extends through a slot in a clutch control rocking arm 19 which depends from a part of the clutch assembly 5 and is secured to the control lever 8 to interlock the control lever 8 with the rocking arm 19 whereby the rocking arm 19 is rotated in response to the rotation of the control lever 8.

The details of the above-mentioned clutch assembly 5 are shown in co-pending U.S. patent application Ser. No. 375,476, filed July 2, 1973.

By the operation of the clutch assembly 5 which is controlled by the rocking arm 19 which in turn rotates in response to the rocking movement of the control lever 8, the sewing machine in which the device of the invention is incorporated will assume the following three positions.

In the first position, the control lever, rocking arm and clutch plate assume the positions as shown in FIG. 3 wherein the transmission of rotational movement from the pulley 6 to the main shaft 1 is interrupted and as result, the sewing machine ceases its operation.

In the second position, the control lever, rocking arm and clutch plate assune the positions as shown in FIG. 4 wherein the rotation speed of the pulley 6 is reduced and the reduced rotational movement is transmitted to the main shaft 1 to drive the sewing machine at a low speed.

In the third position, the control lever, rocking arm and clutch plate assume the positions as shown in FIG. 5 wherein the rotational movement of the pulley 6 is directly transmitted to the main shaft 1 to drive the sewing machine at a high speed.

A spring 18 has one end secured to the control lever 8 and the other end secured to a stationary part of the sewing machine (not shown) so that the control lever 8 is normally urged in the clockwise direction as shown in FIG. 2. A support 23 the entire structure of which has a C-shape is secured to the control lever 8 by means of a bolt or the like fastening means, Thesupport 23 has a pair of vertically spaced upper end lower flanges through which a shaft 22 extends vertically. A stopper lever 24 has one end or the base provided with two spaced flanges positioned between the upper and lower flanges of the support 23 and is rotatably mounted on the vertical shaft 22. The base of the stopper lever 24 had also an engaging piece or pawl 25 secured thereto by suitable fastening means. Reference numeral 26 denotes a leaf spring which has one end secured to the control lever 8 and the other end abutting against the stopper lever 24.

In FIG. 1, a control cam 27 is shown as being interlocked with the main shaft 1 and adapted to be rotated together with a workpiece feed cam 63 in the direction of arrow A at a speed lower than that of the main shaft 1 so as to control the control lever 8. The sewing machine performs its predetermined sewing cycle operation during one complete revolution of the two cams 27 and 63. The peripheral surface of the control cam 27 is formed with an arcuate higher portion adapted to contact a roller 34 (of which description will be made later) during the time the sewing machine is being driven at a high speed, a deeper recess 28 adapted to contact the roller 34 when the sewing machine is in its rest position and two spaced shallower recesses 29A, 293 on the opposite sides of the deeper recess 28 and adapted to engage the roller 34 during the time the sewing machine is being driven at a low speed. An L- shaped interlocking member 31 and an operation lever 32 are mounted in the framework 2 by means of a common stepped screw 30 for movement relative to each other. The interlocking member 31 has one arm which rotatably supports the roller 34 at the free end thereof and another arm which is formed with an arcuate slot 62. The free end of the other arm of the interlocking member 31 is formed with a hook or notch 33 adapted to engage the engaging pawl 25. The action of the spring 18 on the control lever 8 urges the roller 34 against the peripheral surface of the control cam 27 through the control lever 8, stopper lever 24 and interlocking member 31.

A second stepped screw 61 is threaded in the operation lever 32 and extends through slot 62 in the interlocking member 31. A blocking member is secured to the framework 2 by means of suitable fastening means and has one end with which a stopper 59 is integrally formed for limiting the rotational movement of the operation lever 32. A chain 35 is connected at the upper end thereof to the free end of the operation lever 32 and the lower end of the chain (not shown) is connected to an operators treadle.

The thread cutting mechanism of the sewing machine is more particularly shown in FIGS. 3A, 5A, 5B and 6.

A pivot plate or member 36 having three integral arms A, B and C is pivoted in the center to the rear end face of the framework 2 by means of a stepped screw 37. The free end of the arm A rotatably supports a roller 38 in such a manner that the roller contacts the peripheral surface of the thread cutting cam 3. The arm B of the pivot plate rotatably supports a roller 39 at the free end thereof and the arm C is pivoted at the free end thereof to the upper end of a vertical rod 40. The lower end of the rod 40 is connected to an L-shaped link 41 (FIG. 6) which is in turn pivoted to the inner surface of the rear and lower portion of the framework 2 by means of a stepped screw 53 whereby the movement of the pivot plate 36 is transmitted to the link 41 (see FIGS. 3A and 6). A spring 52 extends between a spring anchor 51 secured to the framework 2 and a pin 50 on the link 41 as more clearly shown in FIG. 6. As shown in FIG. 6, a lever 45 is positioned under the needle plate 43 and an auxiliary lever 45 is suitably secured to one end of the first-metnioned lever 45. However, it is to be understood that the two levers can be integrally formed as a unitary structure without departing from the scope of the invention. The thus connected lever assembly 45, 45 is pivoted to the needle plate 43 by means of a stepped screw 44. A rod 42 is connected at one end to the other end of the lever 45 and the other end of the rod 42 is pivoted to the link 41 whereby the movement of the link 41 is transmitted to the levers 45, 45.

A movable thread cutting knife (or moveable thread cutting blade) 47 is positioned under the needle plate 43 and pivoted to the underside of the needle plate by means of a stepped screw 46. A link 48 is pivoted at the opposite ends thereof to auxiliary lever 45 and thread cutting knife 47 whereby the movement of the lever 45 is transmitted to the movable thread cutting knife 47. A stationary thread cutting knife or blade 49 is secured to the underside of the needle plate 43 to shear a thread (not shown) in cooperation with the movable blade 47.

As shown in FIGS. 3A, 5A and SE, a pushing member 56 is suitably secured to the control lever 8 and formed with an engaging face 54 and a recess 55 which cooperate with the roller 39 on the arm B. The engaging face 54 and recess 55 cooperate with the roller 39 in the manner described below. The engaging face 54 continues to push up the roller 39 during the time period of three-fourths rotation of one complete revolution of the main shaft prior to the stoppage of the sewing machine. This serves to hold the roller 38 on the arm A apart from the peripheral surface of thread cutting cam 3. The recess 55 releases the roller 39 from its pushedup position from the time of 3/4 rotation of one complete revolution of the main shaft prior to the stoppage of the sewing machine to the time the sewing machine is in its rest position. This causes the roller 38 to contact the peripheral surface of the thread cutting cam 3.

The entire sewing machine drive control device of the invention operates as follows: FIGS. 1, 2, 3 3A and 7 show the sewing machine in its rest position. When it is desired to drive the sewing machine from the rest position, the operator momentarily steps down the treadle (not shown) connected to the operation lever 32 whereupon the lever 32 pivots in the clockwise direction about the screw 30 as shown in FIG. 1 until the lever abuts against the stopper 59 on the blocking member 60. Such pivotal movement of the operation lever 32 is transmitted through the stepped screw 61 to the interlocking member 31 resulting in the clockwise pivotal movement of the member 31 about the screw 30.

The clockwise pivotal movement of the interlocking member 31 lifts up the roller 34 at its one end to remove it from the recess 28 in the control cam 27 to move to the position in which the roller 34 comes to engage in the recess 29A. At the same time, the hook or notch 33 in the other arm of the interlocking member 31 pushes down the engaging piece 25 secured to the engaging lever 24 whereupon the engaging lever 24 is moved and at the same time, the control lever 8 fixedly connected to the engaging lever 24 pivots by a small angular distance in the counter-clockwise direction about the shaft 7 as seen in FIG. 2. As a result, the stopper pawl 12 which had been engaged in the notch 11 in the stopper member 4 clears the notch 11 as shown in FIG. 4. The pivotal movement of the control lever 8 in the counterclockwise direction moves the pushing member 56 secured to the control lever 8 so as to cause the engaging face 54 on the pushing member to push the roller 39 on the pivot plate 36 upwardly so as to move the roller 38 away from the peripheral surface of the thread cutting cam 3. The rocking arm 19 is slightly moved in response to the pivotal movement of the control lever 8 as shown in FIG. 4 whereby the rotational movement of the pulley 6 is transmitted to the main shaft 1 through the clutch assembly 5 at a reduced rate. Thus, the sewing machine is initially driven at a low speed and the cam 27 starts to rotate in the direction of arrow A at the same low speed as the sewing machine. As the cam 27 rotates in the direction of arrow A, the roller 34 at one arm of the interlocking member 31 contacts the recess 29A in the cam 27 and pushes the cam in response to the rotation of the cam. While the roller 34 is in contact with the recess 29A, the lever 8 is maintained in the position in which the sewing machine is driven at a low speed. As the cam 27, interlocking with the sewing machine, rotates further, the roller 34 is pushed up from the recess 29A onto the nonrecessed portion of the peripheral surface of the cam 27. As a result, the interlocking member 31 is further pivoted in the clockwise direction to move the control lever 8 to the position as shown in FIG. 5. Such movement of the control lever 8 pivots the rocking arm 19 further whereby the rotational movement of the pulley 6 is transmitted to the main shaft 1 through the clutch assembly 5. As a result, the sewing machine is continuously driven at a high speed while the roller 34 is on the non-recessed portion of the peripheral surface of the control cam 27 to effect a cycle operation to form a seam.

Just before the cyclic seam formation operation is completed or just before the control cam 27 completes one complete revolution, the roller 34 on the interlocking member 31 rides on and moves along the recess 29B in the control cam 27 whereby the interlocking member 31 is pivoted about the screw 30 in the counter-clockwise direction as seen in FIG. 2. As a result, the hook or notch 33 in the other arm of the interlocking member 31 releases its oppressive force from the engaging piece 25 to allow the control lever 8 to pivot from the position in FIG. 5 to the position in FIG. 4 by the action of the spring 18. As the control lever 8 moves in the manner mentioned above, the rocking arm 19 is pivoted so as to transmit the rotational movement of the pulley 6 to the main shaft 1 through the clutch assembly 5 at a reduced rate so as to drive the sewing machine at a low speed.

Therefore, before the main shaft 1 rotates about onefourth of one complete revolution prior to the stoppage of the sewing machine in a predetermined position, the roller 34 on the interlocking member 31 drops into the recess 28 in the cam 27. As the interlocking member 31 is further pivoted in this wau, the hook 33 of the interlocking member 31 clears the engaging face and as a result, the control lever 8 is moved from the position in FIG. 4 to the position in FIG. 5B. This pivotal movement of the control lever 8 interrupts the transmission of the rotational movement of the pulley 6 to the main shaft 1 and prepares for thread cutting and stopping of the sewing machine in the predetermined position. More particularly, as the rocking arm 19 which is now in the position of FIG. 4 further pivots in response to the pivotal movement of the control lever 8 to interrupt the transmission of the rotational movement of the pulley 6 to the main shaft 1, the stopper pawl 12 contacts the peripheral surface of the stopper member 4 to prepare for the stoppage of the main shaft 1. As the pusher member 56 moves together with the control lever 8, the roller 39 on the pivot plate 36 is released from the engaging face 54 on the pusher member 56, and roller 39 is positioned above the recess 55 in the pusher member and thus, the roller 38 is caused to contact the peripheral surface of the thread cutting cam 3 by the force of the spring 52 acting on the rod 40. The main shaft 1 rotates about three-fourths of one complete revolution from the position of FIG. SE to the position of FIG. 3A by the force of inertia.

During the rotation of one complete revolution of the main shaft 1 by the force of inertia, the thread cutting mechanism operates to shear the thread and the main shaft 1 then stops in its predetermined position.

During the rotation of the main shaft 1, when the roller 38 moves into the recess in the thread cutting cam 3, the thread cutting blade 47 is pivoted about the stepped screw 46 in the clockwise direction through the rod 40, link 41, rod 42, lever 45 and link 48. During this clockwise pivotal movement of the blade 47, the

end of the blade 47 moves into a loop of the thread carried on the needle and continues to move toward the end of such clockwise pivotal movement so that the thread of the loop and the bobbin thread will be introduced into a hook portion which is formed on the left side of the blade end and has a slot provided with a cutter. Thereafter, as the roller 38 is forced out of the recess in the thread cutting cam 3, the thread cutting blade 47 pivots in the counter-clockwise direction. During this counter-clockwise pivotal movement of the blade 47, the blade 47 cooperates with the stationary blade 49 to shear the thread of the loop and the bobbin thread carried with the hook portion of the blade 47. Thus, the blade actually cuts the thread during movement in the counter-clockwise direction. After the thread cutting operation as mentioned just above, the

stopper pawl 12 engages in the groove 11 in the stopper member 4 as shown in FIG. 3A to stop the main shaft 1 in the predetermined position.

The lever 24 is adapted to stop the sewing machine while the machine is performing the cycle of a seam forming operation. For this purpose, the lever 24 is urged rightwards against the bias force of leaf spring 26 (as seen in FIG. 7) while the sewing machine is being driven at a high speed. When urged rightwards, the lever 24 pivots about the shaft 22 to disengage the engaging piece 25 on the lever 24 from the hook 33 on the interlocking member 31 whereupon the control lever which is now in the position of FIG. 5A is urged in the clockwise direction by the force of the spring 18 to the position of FIG. 3 in which the lever is rest. Thus, the hook 33, engaging piece 25 and lever 24 constitute a shut-off mechanism for shutting off the control lever 8 from the coupling with the interlocking member 31 for stopping the sew machine while the machine is being driven at a high speed.

The afore-mentioned mechanism which transmits the rotational movement of the electric motor to the sewing machine at the same speed as the motor, at a reduced speed and which interrupts this transmission movement may be substituted for any other conventional mechanism. For example, such a mechanism may be the belt shifting-type mechanism as disclosed in US. Pat. No. 3,216,381. The mechanism may also be the so-called pole-change motor in which the number of rotations of an electric motor per unit time is varied by varying the number of poles. When employed in the control device of the invention, the belt shifting mechanism or pole change motor is controlled in relation to the pivotal movement of the interlocking member or control lever 8.

The afore-mentioned mechanism which is adapted to stop the sewing machine in its predetermined position may also be substituted for any other conventional mechanism. For example, the mechanical locking system 29 as disclosed in US. Pat. No, 3,216,381 may be employed for the purpose.

Although only one embodiment of the invention has been illustrated and described in detail, it is to be expressly understood that the invention is not limited thereto. Various changes can be made in the arrangement of parts without departing from the spirit and scope of the invention as the same will now be understood by those skilled in the art.

What is claimed is:

1. A control device for a sewing machine having a frame, primary mover means and a main shaft, said control device comprising:

a control cam interlocked with said main shaft for rotation in one direction at a speed lower than that of said main shaft, said control cam having a cam surface including a cylindrical surface portion, a recess portion, and first and second cam portions, each cam portion having a cam surface extending from said recess portion to said cylindrical surface portion with the distance between an axis of the r0- tation of said control cam and said cam surface being gradually increasing from said recess toward said cylindrical surface portion;

an interlocking member pivotally mounted on said frame of said sewing machine and having a first end carrying a follower roller in contact with said cam surface of said control cam, said interlocking member being reciprocally pivotable while said roller is in contact with said cam surface of said control cam;

a control lever rotatably mounted on said frame of said sewing machine and coupled with a second end of said interlocking member, said control lever being reciprocally pivotable about an axis in response to pivoting movement of said interlocking member, said control lever being shiftable about said axis by said interlocking member among a first position when said follower roller is in said recess portion of said cam surface of said control cam, to a second position when said follower roller is in contact with said cam portion of said control cam, and a third position when said follower roller is in contact with said cylindrical surface portion of said control cam;

clutch means for transmitting and interrupting power from said primary mover means to said main shaft, said clutch means being operatively coupled with and controlled by said control lever in such a manner that when said control lever is in said first position, the power transmission is interrupted, and when said control lever is in said second position, a low-speed transmission ofpower is effected, and when said control lever is in said third position, a

at a high speed.

3. A control device as claimed in claim 2, wherein said shut-off mechanism comprises a stopper lever mounted on said control lever, an engaging piece on said stopper lever and a hook on said interlocking member, said hook normally engaging said engaging piece, but being disengageable therefrom when said stopper lever is operated when said sewing machine is to be stopped from its driven position so as to position said control lever in a position in which the sewing machine is held in its rear position. 

1. A control device for a sewing machine having a frame, primary mover means and a main shaft, said control device comprising: a control cam interlocked with said main shaft for rotation in one direction at a speed lower than that of said main shaft, said control cam having a cam surface including a cylindrical surface portion, a recess portion, and first and second cam portions, each cam portion having a cam surface extending from said recess portion to said cylindrical surface portion with the distance between an axis of the rotation of said control cam and said cam surface being gradually increasing from said recess toward said cylindrical surface portion; an interlocking member pivotally mounted on said frame of said sewing machine and having a first end carrying a follower roller in contact with said cam surface of said control cam, said interlocking member being reciprocally pivotable while said roller is in contact with said cam surface of said control cam; a control lever rotatably mounted on said frame of said sewing machine and coupled with a second end of said interlocking member, said control lever being reciprocally pivotable about an axis in response to pivoting movement of said interlocking member, said control lever being shiftable about said axis by said interlocking member among a first position when said follower roller is in said recess portion of said cam surface of said control cam, to a second position when said follower roller is in contact wIth said cam portion of said control cam, and a third position when said follower roller is in contact with said cylindrical surface portion of said control cam; clutch means for transmitting and interrupting power from said primary mover means to said main shaft, said clutch means being operatively coupled with and controlled by said control lever in such a manner that when said control lever is in said first position, the power transmission is interrupted, and when said control lever is in said second position, a low-speed transmission of power is effected, and when said control lever is in said third position, a high-speed transmission of power is effected; and an operation lever means operatively connected to said interlocking member for selectively shifting said control lever from said first position to said second position for starting said sewing machine.
 2. A control device as claimed in claim 1, further comprising a shut-off mechanism means interconnected to said control lever and said interlocking member for selectively removing said control lever from the coupling engagement with said interlocking member for returning said control lever from said third position to said first position while the main shaft is being driven at a high speed.
 3. A control device as claimed in claim 2, wherein said shut-off mechanism comprises a stopper lever mounted on said control lever, an engaging piece on said stopper lever and a hook on said interlocking member, said hook normally engaging said engaging piece, but being disengageable therefrom when said stopper lever is operated when said sewing machine is to be stopped from its driven position so as to position said control lever in a position in which the sewing machine is held in its rear position. 